Packet-switched networks, such as the Internet, are increasingly being used to carry traffic, such as voice calls, that have traditionally used circuit-switched networks, such as the public switched telephone network (PSTN). This shift is being motivated by a number of different factors. For example, packet-switched networks may allow voice traffic to be integrated with data, video, and other traffic. Packet-switched networks are typically less expensive to build and maintain than circuit-switched networks, and packet-switched networks often have higher bandwidth than is available in most circuit-switched networks. However, integration between circuit- and packet-switched networks is important because packet-switched networks are likely to co-exist with circuit-switched networks for some time.
Circuit-switched voice traffic calls are typically established using the Public Switched Telephone Network (PSTN), which makes use of the Signaling System #7 (SS7) protocol. In order to initiate a circuit-switched call, a source client device typically originates a call by specifying a PSTN number associated with a destination client device to a network switch associated with source client device. The network switch associated with the source client device transmits to the circuit-switched network an Integrated Service Digital Network User Part (ISUP) Initial Address Message (IAM) containing the PSTN number. The ISUP IAM is routed through the circuit-switched network to a network switch associated with the destination client device. The network switch associated with the destination client device then attempts to contact the destination client device to initiate the call.
By contrast, packet-switched voice traffic calls, such as Voice over Internet Protocol (VoIP) calls, are typically established using the Internet and may make use of the Session Initiation Protocol (SIP). In a packet-switched call, a source client device typically originates a call by specifying a VoIP address associated with a destination client device to a SIP proxy associated with the source client device. The SIP proxy associated with the source client device transmits to the packet-switched network a SIP INVITE message containing the VoIP address. The SIP INVITE message is routed through the packet-switched network to a SIP proxy associated with the destination client device. The SIP proxy associated with the destination client device then attempts to contact the destination client device to initiate the call.